Wireless communication networks using wireless terminals and wireless base stations have become widespread, and communication services that allow a wireless terminal to perform wireless communication anytime, anywhere and with anyone are provided today. Further, because the contents of communication services evolve day by day, a wireless base station needs to update software according to need. At this time, the wireless base station carries out a reset. Further, in the event of a defect occurring while providing a communication service, the wireless base station carries out a reset in order to clear this condition in some cases. When the wireless base station carries out a reset, it narrows the cell range (a cell is the range reached a radio wave of the wireless base station) (or reduces the transmission power). In addition, the wireless base station narrows the cell range (or reduces the transmission power) in some cases, not limited to when carrying out a reset.
However, in the case where a wireless base station narrows the cell range (or reduces the transmission power), various problems can occur. For example, a call disconnection can occur in a wireless terminal that has made a call connection to a wireless base station that has narrowed the cell range (or reduced the transmission power). Therefore, a technique to avoid the occurrence of a call disconnection in the case of narrowing the cell range (or reducing the transmission power) has been proposed recently. For example, Patent Literatures 1 and 2 propose techniques to avoid the occurrence of a call disconnection in the case of reducing the transmission power for a software update.
According to the technique proposed in Patent Literature 1, a wireless base station gradually reduces the transmission power at the time of a software update, so that a wireless terminal that has made a call connection to the wireless base station is handed over to an adjacent wireless base station that is located adjacent to the wireless base station. It is thereby possible to avoid the occurrence of a call disconnection.
Further, according to the technique proposed in Patent Literature 2, a wireless base station reduces the transmission power and eliminates the overlap with the cell of an adjacent wireless base station, switches a communication channel during provision of a communication service to the adjacent wireless base station, and carries out a software update in this state. It is thereby also possible to avoid the occurrence of a call disconnection.
The techniques disclosed in Patent Literatures 1 and 2 are based on the assumption that the cell of a wireless base station that reduces the transmission power overlaps the cell of an adjacent wireless base station. In reality, however, the cell of a wireless base station includes an area that does not overlap the cell of an adjacent wireless base station. If the wireless base station reduces the transmission power in the circumstances where a wireless terminal is located in such an area, that wireless terminal cannot be handed over to the adjacent wireless base station, and thereby a call disconnection occurs. This issue is described hereinafter with reference to FIG. 1, using an example of an LTE (Long Term Evolution) wireless communication system which is widely used as the communication standard of a cell-phone unit.
The cell range of a wireless base station is generally several km, and the coverage area is limited with one wireless base station. Therefore, in an actual wireless communication network, a plurality of wireless base stations (which are eNBs (evolved NodeBs) in the case of the LTE) are placed as shown in FIG. 1 so as to expand the area where a wireless terminal can communicate.
In FIG. 1, seven eNBs (#1-#7) 10-1 to 10-7 are placed, and the eNBs (#2-#7) 10-2 to 10-7 are adjacent to the eNB (#1) 10-1. Cells of the eNBs (#1-#7) 10-1 to 10-7 are referred to as cells (#1-#7) 10-1a to 10-7a, respectively. Further, when a wireless terminal that has communicated with the eNB (#1) 10-1 in the area of the cell (#1) 10-1a moves to the area of the cell (#2) 10-2a, the wireless terminal switches the wireless base station with which it communicates from the eNB (#1) 10-1 to the eNB (#2) 10-2. This operation of switching is called a handover, and by carrying out a handover, the wireless terminal can continue communications without a call disconnection even when it moves to a different cell.
For example, it is assumed that the eNB (#1) 10-1 gradually reduces the transmission power. In this case, a wireless terminal that is located in the shaded area that overlaps the cells (#2-#7) 10-2a to 10-7a of the eNBs (#2-#7) 10-2 to 10-7 that are adjacent to the eNB (#1) 10-1 can be handed over to an eNB corresponding to the overlapping cell. However, a wireless terminal that is located in the blank area of the cell (#1) 10-1a cannot be handed over because a radio wave does not reach from any of the eNBs (#2-#7) 10-2 to 10-7, and a call disconnection occurs.